Originally Posted by MILD THUNDER (Post 4488903)

Heres a thread I had started about tims combo, in another forum. If anyones interested.

http://www.speedtalk.com/forum/viewt...hp?f=1&t=47746

Originally Posted by Newold1 (Post 4488903)

the camshaft calculations show a recommended intake duration of 252 @ .050 and exhaust at 262@ 050" with 1.8 ratio rockers and let the rockers do the extra lift

Originally Posted by getrdunn (Post 4489196)

ok so from beginning to date of this thread there has been a lot discussed. Some off the subject in question but mostly involving input with combos, cam specs, lobe flaws with premature failure etc. To me it's evident something has changed drastically over the last couple years with rmbuilder but perhaps much longer. Difficult to say as we don't have all input going back several years (decade or more). Especially when we upgrade our builds, sell engines and lose track of the long jevity of the said engine and valvetrain. Just saying we're constantly changing builds, going bigger, adding SC's, procharger's, tunnel rams and so on which in most cases involves cam and valvetrain in most cases. So in a nutshell who's to say how long who's cam last and who's doesn't.

Other thread MT started may shed shed some more light on that. List is growing without a doubt. Good bad indifferent.

What I have come to the conclusion for the most part here is without a doubt duration goes up with cubic inch to maintain peak hp at a given rpm. (Pretty common knowledge for most part). Also obviously short duration higher lift cams are causing valvetrain stability all over. You'd think after all the years of techknowledgey and R&D there would just be a simple ratio rule of thumb (lobe/duration). Yet most likely there is that's why you don't see cams similar to the ones in question being sold by most all cam companies.

I will have to admit I am shocked to learn how much duration is needed to achieve peak hp let's say 6,000-6,200 on a 540-548. What I am understanding for the most part is on a 540-548 build with 315-325 cc heads is you should be looking at a cam 250/260-700/690 plus or minus. Heads being 100% professionally set up is crutial. If in doubt send them out... Every single moving part pertaining to valvetrain including lifter bore alignment, size, etc all equally important.

I recall MT posting some CI's vs duration differences but would be kinda of neat to really narrow it down with most common builds. Runner cc can change up duration so formula would never be a 100% accurate but at least get many people in the ball park. Head runner 305-335 (probably average based on CI below)

Just keep 6,000 rpm as a guideline for all engines.


496-509

540-548

555-565

572-598

Some variables may apply such as same CI with different stroke but just looking for general duration. Might be best for another thread however I think would be helpful for many individuals as well as builders. Example when you look up a cam in cam book it's not real specific with 396-454 or some 396-502. I have seen 540 plus occasionally however I think what we could do here would be helpful. Lift rates easier to determine based heads and flow at various lifts.


As many cam threads that come up I think this would be helpful for many.

Originally Posted by JRider (Post 4489226)

I am not so sure about that small of runner head on a 540+CI marine engine. My 548s have Dart 360s speced by Teague and made great numbers. Bob recommended AFR 305s for my 502ci 575sci, I thought that was small as well, or was it just what he had on the shelf? As far as my cams they are Crane Hydraulic Roller 254/262 680/680. 982 and 984 a side at 8psi

Originally Posted by bck (Post 4489231)

I've got 305s on my 548 and I'm happy with the power they are making, I feel it was a good choice considering what it costs to move up. If I want to make more power in the future I think I'd have to go bigger.

Originally Posted by bck (Post 4489241)

If I could do it over again no doubt. Seems like it'd be much easier. The only reason I didn't was because I had just paid to remove the blower scoops and fill in the opening in the bulkhead for the drive belt. Oops

Originally Posted by JRider (Post 4489226)

I am not so sure about that small of runner head on a 540+CI marine engine. My 548s have Dart 360s speced by Teague and made great numbers. Bob recommended AFR 305s for my 502ci 575sci, I thought that was small as well, or was it just what he had on the shelf? As far as my cams they are Crane Hydraulic Roller 254/262 680/680. 982 and 984 a side at 8psi

Originally Posted by getrdunn (Post 4489244)

Nice combo especially for a supercharged cat. That thing must scoot... Now that size of runner in a heavy deep V NA app would be very questionable. How fast you running? Also meant to ask you what rpm your propped for?

I did see the cam 12.1 comp min recommended. Obviously with the numbers you pulled it didn't apply to your build.